# Density, Viscosity, and Buoyancy

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Density, Viscosity, and Buoyancy
7A investigate and identify properties of fluids including density, viscosity, and buoyancy;

What is Viscosity? A measure of a material’s resistance to flow
Which substance here has a higher viscosity? The Ketchup has a high viscosity compared to the water. Viscosity is a property of fluids, along with density and buoyancy. Viscosity is the measure of a material’s resistance to flow. In this example, catsup has a higher viscosity, since it resists flow much more than water. This is something that many people have actually experienced. It may be fun to ask people to try and give examples of materials that they see everyday that have high viscosity. Some may include molasses, honey, certain kinds of glue, nacho cheese dip, some kinds of paint, and many other materials. If we could measure how fast the liquid actually moves, we could get a better idea of a material’s viscosity. However, this could get messy and the equipment we need to use like the photogates and the timer don’t get along too well with liquids. What we need to do is keep the liquids contained and keep our work area and equipment free from sticky substances. Luckily, there is an easy, safe and fun solution to this problem. The Viscometer.

Seeing the Effects of Viscosity
Substances with higher viscosity take longer to flow down the ramp. Viscosity is a measure of the resistance that one layer of liquid experiences when flowing over another layer.

Measuring Viscosity Viscosity can be measured by how fast a object falls through a liquid.

Is there a relationship? Low density indicates low viscosity. High density indicates high viscosity. This is not always true. Think of water and oil. Oil is more viscous but less dense. Think of water and corn syrup. Corn syrup is more viscous but corn syrup has a greater viscosity. Density actually measures how tightly packed the molecules in a substance are. Viscosity measures resistance to flow. Its not a huge stretch to imagine there may be a correlation. If something is tightly packed it could conceivably resist flow more that the same molecules less tightly packed, and this is quite true when we compare the liquid and gas state of the same materials. Gasses are much less resistant to flow. However, different substances do not have the same sized or shaped molecules, and that matters a great deal. We really are not comparing the same size or shape of molecule with density, only the number of them. The converse holds true for viscosity – we are concerned with the size and shape, but not so much the number of molecules. If two substances have similar shaped and sized molecules, but the density varies greatly, then we may use the fact that less tightly packed molecules would resist flow less. This concept can be applied to different kinds of oil that may similar molecules but different densities. But if we don’t really know the size and shape, density really may not be the greatest indicator of a substance’s viscosity.

Size and Viscosity Which molecule would be easier to pour from a bottle? Water or oil Which would experience more friction? How does that affect viscosity? More friction, greater viscosity Longer molecules have more friction. When looking at the molecules on the slide it is easy to see that the oil molecule is much more likely to become entangled with other oil molecules when being poured. This is vital to viscosity. Consider the shape of these two molecules. The oil molecule is quite long and somewhat thick. The water molecule in comparison is compact and small. Will they actually pour? Experience tells us yes. Being poured is pretty much like rolling downhill. The molecules bump into one another as they tumble out of the bottle. If we think of the oil as being shaped close to a big thick tree and the water like a almost round boulder, which one would roll downhill easier? The boulder. Why? The big heavy tree would experience more friction. Now imagine billions of big oil molecules trying to roll down at the same time – Log Jam, lots of friction and the process would take a while. In other words, it would resist flow. A bunch of roundish boulders on the other hand would make for a pretty easy roll downhill and in fact landslides and avalanches are a real problem in some parts of the world. The boulders would resist the flow much less than logs with big branches on them.

Application of Viscosity
Viscosity if the most important property of motor oil. Viscosity depends on temperature – Increase temperature will decrease the viscosity of the material. In the winter you need a lower viscosity oil so the oil will flow better. In the summer, you need a higher viscosity oil so during the higher summer temperatures it will not flow too fast. (If an oil is too thin it does not adhere to the surfaces and lubricate the moving parts.)

Motor Oil SAE – Society of American Engineers
The viscosity of oil tends to decrease as it heats up (oil break downs) Big molecules to little molecules. What does 10 W 30 mean? Meets viscosity requirements for winter (low temp) Viscosity index – hot engine Viscosity Index- Cold Engine

Viscosity Review Viscosity is the resistance to flow.
For example pancake syrup flows more slowly than water. We say the pancake syrup is more viscous. Ketchup and molasses are good examples of “viscous” materials. Viscosity is the most important property of motor oil. Increase temperature will decrease viscosity of a material.

Density Density is a relationship between mass of a substance and its volume. Water has a density of 1 More dense substances sink and less dense objects float If you add salt to water it becomes more dense If you heat a substance it becomes less dense. D = M V

Density As you heat a substance its molecules move farther apart and the substance becomes less dense. Ice Liquid Gas

Buoyancy The upward force on a body immersed or partly immersed in a fluid.

1. What is the density at 20C of 12
1. What is the density at 20C of 12.0 milliliters of a liquid that has a mass of 4.05 grams? 0.338 g/mL 2.96 g/mL 16.1 g/mL 48.6 g/mL

Powerful Plankton The U.S. Naval Research Laboratory has created an experimental marine fuel cell that could produce enough electricity to power ocean-monitoring devices. This fuel cell runs on seawater and sediment, with the help of plankton. Some plankton on the surface of ocean sediments use dissolved oxygen to break down organic matter, releasing energy; this is an aerobic process. The plankton in the deeper sediments break down organic matter without using oxygen; this is an anaerobic process. These two processes create a difference in voltage between the surface of the sediment and the sediment farther down in the seabed. The voltage difference can be used to produce electricity—up to 5.0V; 1.0 – 2 watts of power. Energy supplied by this type of fuel cell can be obtained as long as there is organic matter in the sediment.

2. What is the mass of a 500.00 mL sample of seawater with a density of 1.025 g/mL?

3. Which of the following objects will float on water?
[Option 1] [Option 2] [Option 3] [Option 4]

4. Students in a chemistry lab measure the time it takes four different 100 mL solutions to pass through a hole in the bottom of a cup. Which of the following properties of the solutions is most likely being measured? Buoyancy Mass Viscosity Volume

Watersheds The San Antonio–Nueces Coastal Basin, shown in the map below, consists mainly of flatland. The soil in the area is mostly sandy and porous. Rainfall that collects in this area drains into the Gulf of Mexico by a series of waterways. The San Antonio–Nueces Coastal Basin is one of more than 20 major watersheds in Texas. A watershed is a geographic area that drains water collected on its surface. Watersheds provide a way for water to move from areas of higher elevation to areas of lower elevation. Sediment and dissolved materials are also moved to lower elevations. Rivers and streams form a general pattern within a watershed. The map shows this general pattern within the San Antonio–Nueces Coastal Basin. This type of pattern is called a dendritic pattern and can be observed in many natural structures, including the network of nerves in the human nervous system and branching patterns in trees. Watersheds can change as the geography and other features of an area change. Some of the causes of change are more obvious than others. For example, the clearing of wooded areas for the construction of buildings may change the way rainfall collects and drains from an area.

5. Use the information on the previous page to answer this question
5. Use the information on the previous page to answer this question. A sample of water from a stream has a greater density than a sample of distilled water. Which is the best explanation for the difference in density? Streams have kinetic energy. Distilled water has a neutral pH. Streams can contain dissolved salts. Distilled water can contain dissolved gases.

6. From the data in the table, what would be the mass of a 3
6. From the data in the table, what would be the mass of a 3.0 L sample of water collected from Gunnison Bay? 2515 g 3171 g 3579 g 4193 g

7. A diet cola and a regular cola in identical cans are each placed in separate beakers containing water. Which of the following conclusions is the best for the experiment shown above? The diet cola is less dense than the regular cola. The diet cola has fewer calories than the regular cola. There is more buoyant force on the can of diet cola than on the regular cola. There is more liquid in the can of diet cola than in the regular cola.